The interpretation of the observations on the clearances of inulin and diodrast, as measures of rate of glomerular filtration and of renal plasma flow, has been handicapped by lack of quantitative criteria of changes in afferent and efferent renal arteriolar resistance. Most of the qualitative criteria suffer because they do not take account of simultaneous changes in blood pressure, and because they over-simplify by ascribing the effect either to the afferent or the efferent arterioles alone. Where so many factors are interrelated, as in the components determining renal arteriolar resistance, mathematical relationships are necessary. We have previously offered formulas for renal arteriolar resistance (1, 2). They have here been applied to data on renal function in dogs, and also in man, published by Corcoran and Page et al. (3 to 7), whom we wish to thank for their kindness in putting at our disposal supplemental and unreported observations.3 We are also grateful to Mr. Julius Stein for his painstaking aid in computation.